Anaerobic Digestion 101

What is anaerobic digestion?

Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen.

Is it a new technology?

Not exactly. Anaerobic digestion has naturally occurred for tens of millions of years. The first anaerobic digester was built in India in the 19th century and we continue to improve the technology to meet the demands of our culture.

What materials do you put into an anaerobic digester?

Ideal materials – also known as feedstocks – for digesters include:

  • food waste
  • municipal wastewater solids
  • fats, oils and grease (FOG)
  • livestock manures

High-caloric feedstocks with lots of fat, sugar and starch typically yield more biogas than other materials.

Why do communities choose to use anaerobic digestion technology?

Assuming those communities have already implemented other options that represent the most efficiency for these materials (e.g. food waste reduction, food donation, backyard composting) anaerobic digestion is the logical next step because it adds value to the organic recycling process providing:

  • Energy: Anaerobic digestion provides biogas, a clean, local renewable energy source. 
  • Nutrients: Anaerobic digestion provides nutrient-rich soil amendment sand natural fertilizers that can be returned to local landscapes.
  • Maximum odor control: Anaerobic digestion occurs in gas tight environments.

Biogas, what’s that?

Biogas is a mixture of mostly methane and carbon dioxide with trace amounts of other gases. It can be used to fuel an engine generator for creating renewable electricity and heat, upgraded for injection into a natural gas pipeline, or compressed into vehicle fuel (CNG)

Kids Corner

An 8-year-old clean energy enthusiast interviewed our bacteria mascot, Burp Reynolds. The Question: “How does anaerobic digestion turn pizza crusts into power?”

The Report Back to the Class: “Anaerobic digestion is awesome! You put stuff, like pizza crusts, banana peels and fish guts – into a warm, airtight box.Then tiny bugs eat up the yummy things like the fats, sugars and starches. Then the bugs burp and fart! And if you capture those burps and farts – it’s biogas –you can use it for electricity, heat or fuel. It’s pretty cool.”

What products do you get from anaerobic digestion?

Typically, three things come out of an anaerobic digester for beneficial reuse:
  • Biogas (which can be turned into electricity and heat, or fuel)
  • Solid fertilizer (from the digestate, the materials after they have been digested)
  • Liquid Fertilizer (from the centrate, the liquids after they have been digested)

How would a community use anaerobic digestion?

It depends on the needs of the community. Imagine the possibilities:
  • A municipality might want to send its mixed food waste and yard waste to a high-solids digester like the one we built in Vancouver. Or they might want to send just food scraps to a co-digestion facility at a wastewater treatment plant to optimize biogas yields.
  • A university, institution, or commercial business might want to develop a project for their concentrated generation of on-site organic waste.
  • A food processor or brewer might want a digester to process their organic waste.
  • A farmer might want to add value to their manure waste streams.
  • A community might just need a portion of our platform of technologies,such as depackaging equipment and services.

What kind of anaerobic digester is right for my community or business?

Typically, three things come out of an anaerobic digester for beneficial reuse:

FEEDSTOCK

  • What is the composition? The technical term for this question is around the solids content of the materials: Are they high solids (e.g.“stackable” with a 18-40% solids content) or low solids (e.g.“pumpable” with a 5-18% solids content)?
  • Who are the generators? (e.g. residents, businesses, breweries,food processors, institutions, etc).
  • What types of volumes do you expect? (e.g. tons per month;any seasonal variation)
  • How much will it need to be pre-processed (e.g. will any packaging or contamination like plastic or metal need to be removed?)

FOOTPRINT

  • What type of property is available? (e.g. What is the ownership? Where is it located? How would you characterize the neighborhood and local community?)
  • What is the regulatory environment of the site? (e.g. What permits are required?)

FLOW

  • What flavor of digestion works best given the feedstock and footprint described above? This is where the fun begins. We’ll talk you through the options. Some variables that will arise include temperature (mesophilic or thermophiic), process (batch or continuous) and end product (compost or fertilizer).

Contact Harvest Power today to develop a project.

What does the anaerobic digestion process look like, exactly?

AD_Process

Anaerobic digestion consists of four stages that happen on the microscopic level:

    1. Hydrolysis – Longer chain carbohydrates, fats and proteins are broken into shorter chain molecules
    2. Acidogenesis – Shorter chain molecules produce carbon dioxide, hydrogen sulfide and more volatile fatty acids
    3. Acetogenesis – Simple molecules created through the first two steps are digested by specific bacteria to produce acetic acids as well as hydrogen and carbon dioxide
    4. Methanogenesis – A certain class of bacteria known as methanogens utilizes the products developed in stages 1-3 and converts them into methane, carbon dioxide and trace amounts of other gases